Besides providing the octane level needed for gasoline products, the catalytic reformer is the principal hydrogen producer within a refinery. An important aspect of reformer operation is to generate as much hydrogen as possible, consistent with other requirements, of a quality suitable for use in the hydrogen-consuming units, particularly hydrocrackers and hydrotreaters.
The overhead vapor from the reformer reactors is typically split into at least two portions, one for recycle in the reactor loop, the other that forms a purge from the loop and that is the source of the net hydrogen product. This reactor purge stream is often sent to pressure swing adsorption (PSA) for upgrading to a high purity hydrogen product, typically containing 99% or more hydrogen. The hydrocarbon tail gas stream created when the PSA beds are regenerated is frequently treated as a waste gas stream and sent in its entirety to the plant fuel header.
Use of pressure swing adsorption (PSA) for this upgrading step is taught, for example, in U.S. Pat. No. 5,332,492, in which PSA tail gas is returned to the contactor section of the phase-separation steps, and U.S. Pat. No. 5,278,344, in which PSA is used to purify the net hydrogen stream after catalytic reforming and in front of a hydrodealkylation step.
It is also possible to use PSA to treat other diverse streams containing mixtures of hydrogen and hydrocarbons in refineries and elsewhere. Representative examples of such hydrogen-containing streams include overhead streams from fractionation columns used downstream of catalytic crackers, hydrocrackers and the like, overhead streams from cryogenic and other condensation units, overhead streams from absorbers, effluent streams from steam reformers, and refinery and petrochemical waste streams in general.
It is also known to use membrane separation for removing hydrogen from hydrocarbons in gas streams from various sources. U.S. Pat. Nos. 4,362,613, and 4,367,135, both to Monsanto, describe processes for treating the vapor from phase separators in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the reactor. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; and "Optimizing Hydrocracker Hydrogen" by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons. Papers that specifically concern treatment of reformer off-gases are "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; and "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985. In these papers, a system and process using membranes to treat the overhead gas stream from the absorber/recontactor section of the plant are described. All of these papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063 and 4,836,833, to Air Products and Chemicals, and U.S. Pat. No. 4,892,564, to Cooley. U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
The use of rubbery polymeric membranes operated at low temperature to separate methane from nitrogen is taught in U.S. Pat. No. 5,669,958.
It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
Numerous patents describe combinations of membrane separation with PSA. Representative examples include U.S. Pat. Nos. 4,229,188; 4,238,204; 4,398,926; 4,690,695; 4,701,187; and 4,783,203. U.S. Pat. No. 5,332,424, to Air Products and Chemicals, describes fractionation of a gas stream containing hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Optionally, the membrane separation step is followed by PSA treatment. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,447,559; and 5,507,856, which all show combinations of carbon adsorbent membranes followed by PSA. U.S. Pat. No. 5,634,354 discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen.
U.S. Pat. No. 5,435,836, concerns treatment of mixtures of hydrogen, carbon dioxide, carbon monoxide and methane from steam reformers. The gas mixture from the steam reformer is treated by PSA to recover a high purity hydrogen stream. The waste gas from the PSA unit is then treated by membrane separation using a carbon adsorbent membrane. The hydrogen-rich residue is returned to the PSA unit and the permeate gas from the membrane unit can optionally be used as fuel for the steam reformer. U.S. Pat. No. 5,753,010 discloses a process similar to that of U.S. Pat. No. 5,435,836, but in which the tail gas from the PSA unit is split into two fractions of unlike composition, which are treated separately in two discrete membrane steps.
Patent application Ser. No. 09/083,784, copending with the present application, describes treatment of off-gases from fluid catalytic cracking absorbers using hydrocarbon-selective membranes.
Patent application Ser. No. 09/083,660, copending with the present application, describes treatment of overhead gases in hydrocarbon conversion reactors of any type by passing gases in the reactor recycle loop across hydrocarbon-selective membranes. Patent application Ser. No. 09/083,872, copending with the present application, describes such a process applied specifically to hydrotreaters and hydrocrackers.
Patent application Ser. No. 09/083,560, now U.S. Pat. No. 6,011,192, issued Jan. 4, 2000, describes a process in which a rubbery polymeric membrane is used to condition a gas stream to remove heavy hydrocarbons before PSA treatment.
Patent application Ser. No. 09/083,653, copending with the present application, describes the use of hydrogen-rejecting membranes to directly treat overhead gases from the phase separators of catalytic reformers.